F.e. when you have more and different ParamConverter per entity you want to name them explicitly.
Then you can use named ParamConverters.

In the documentation this issue is a bit fragmented, so here is the compact version:

Declare:
Set the “converter” attribute in the param_converter service tag.
Read it in the documentation: http://symfony.com/doc/current/bundles/SensioFrameworkExtraBundle/annotations/converters.html#creating-a-converter

You can register a converter by priority, by name (attribute “converter”) or both.

<service id="my_converter" class="MyBundle\Request\ParamConverter\MyConverter">
    <tag name="request.param_converter" priority="-2" converter="my_converter" />
</service>

Use:
In you controller annotation call the ParamConverter with the converter attribute.
The Symfony ParamConverter dokumentation explains it like this:
If an explicit converter choice was made with @ParamConverter(converter=”name”) the converter with the given name is chosen.

use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\ParamConverter;

/**
 * @ParamConverter("post", converter="my_converter")
 */
public function showAction(Post $post)
{
}

In some apps however you need to cache data behind the scenes, f.e. responses from API calls or custom objects sets.
Symfony2 itself doesnt have such a functionality on first sight (symfony2 doesnt, but Doctrine, see below) and so I searched for one and first found a bundle which utilize the Zend Cache lib:
https://github.com/KnpLabs/KnpZendCacheBundle

This worked well but as discussed here(https://github.com/KnpLabs/KnpZendCacheBundle/issues/2) this adds dependencies to your Symfony2 project. This is actually not necessary since Doctrine/Commons is almost always part of your Symfony2 distribution and the Doctrine/Commons provides a Cache Layer as well.
A very good one, indeed.

So if you need to cache data use Doctrine/Commons.

This Cache Layer abstracts the caching functionality and provides already various different backends for your caching data.
These are already build-in in the master version:

• APC
• Array
• Filesystem
• Memcache
• PhpFile
• Redis
• WinCache
• Xcache
• ZendData

You could even create your own on top of the CacheProvider class and the Cache interface.

In your Symfony2 project simply register your cache service of choice and your ready to go.
In your config.yml or services.yml add:

cache:
        class: Doctrine\Common\Cache\PhpFileCache
        arguments: [%kernel.cache_dir%]

And in your controller you can call the service and save and load data from the cache.

As you can see you can set a namespace for your cache data, so that you can easy use it for different scenarions in the same app.
Further you can set a time-to-live (TTL) in seconds as third parameter of the save method.

So after all symfony2 has a caching mechanism for data, its just a little hidden in the Doctrine/Commons dependency.

Update from the Update!!
LiipDoctrineCacheBundle is abanndoned, use https://github.com/doctrine/DoctrineCacheBundle
Update!!
————————————————————-

I just was told that there is a Bundle for the Doctrine/Commons cache:
https://github.com/liip/LiipDoctrineCacheBundle

So I recommend to use this.
The benefit of this is, you can configure your caches through yml files and you have one central cache reference. I guess this saves some bytes in case you use different caches with different namepaces.

So with the LiipDoctrineCacheBundle the above example would go like this (given you have already registered the bundle):

config.yml

liip_doctrine_cache:
    namespaces:
        mynamespace:
            namespace: mynamespace.cache
            type: php_file

IndexController.php

$cache = $container->get('liip_doctrine_cache.ns.[mynamespace]');
if (false === ($cached_data = $cache->fetch($cache_key))) {
            $cached_data = $SOMEAPI->getData($params);
            $cache->save($cache_key, $cached_data, 3600);//TTL 1h
}

its much nicer to use the DirectoryIndex in your URL and not having the web_app.php, thats the aesthetics.
But it also avoids having some hassle with ajax urls which are defined absolute in some scripts like
url: /myajax/call.json

The Environment can be changed in the .env.php file which just returns the Environment value:

<?php
return $env = 'dev';

Personally i dont mind changing the Env in the code rather than in the url, while in development.
It might be a tick more far away but its rather uncommon anyway and it would be more effort to change the vhost DirectoryIndex on web_dev.php and prod everytime to get a clean url.
(thanks max & joshi)

2. resolve console and webserver user conflict for cache
Different to Symfony 1.x Symfony2 assumes a more sophisticated User Managment on your machine for Security reasons, which absolutly makes sense. Read more about it here:http://symfony.com/doc/current/book/installation.html#configuration-and-setup
This means you have to adjust the rights for the webserver- and for the console user that both can create and delete the cache.
This requires some admin skills which might be a bit too much for starters.
And some people start wondering why the

php app/console cache:clear

command wont work and start using

rm -rf app/cache/*

in desperation.

To fix this Symfony2 recommends as a ‘hotfix’ to uncomment the

umask(0000);

part on top of your bootstrap files.
So simply uncomment the line in app_dev.php ( if you use the one from above its already done :) ) and in app/console.
Thats alright for local setup, for production go and talk to your admin of trust or live the life of adventure.

3. make a local config that can be placed in gitignore
Here you can store config parameters and overwrite common ones to fit your local setup. Put it in the gitignore so your fellow devs can do the same.
Therefor we need to hack the AppKernel.php and redefine the registerContainerConfiguration method like so.

public function registerContainerConfiguration(LoaderInterface $loader) {

        if ($this->getEnvironment() == 'dev') {
            $extrafiles = array(
                __DIR__ . '/config/config_local.yml',
            );
            foreach ($extrafiles as $filename) {
                if (file_exists($filename) && is_readable($filename)) {
                    $loader->load($filename);
                }
            }
        } else {
            $loader->load(__DIR__ . '/config/config_' . $this->getEnvironment() . '.yml');
        }
}

Read more about it:
http://stackoverflow.com/questions/7338767/can-i-include-an-optional-config-file-in-symfony2/10336451#10336451

Now have fun with bundles & services.