Ok, lets start with a couple of obvious links:
UltraPack:
UltraPack: http://ultrapack.il2war.com/index.php
The most popular online mod pack - some rather large downloads, and they are just about to release version 3.0, so you may do better to wait for that, rather than downloading the current version.
SAS:
http://www.sas1946.com/main/index.php
The site of chouce for the majority of modders, these days. They have their own Modactivator for custom mod setups - preferable if you only want to install a few mods, or like to tinker.
Both sites expect you to have at least at some technical competence regarding computers, and SAS can be a bit snappy at newcomers asking questions that have already been answered - search first. And don't spam the forums with requests to make this or that mod, or asking 'when it will be ready' - the answer is always going to be 'when it is done'.
Even if you aren't actually interested in installing mods yourself, I'd take a look at some of the things that SAS have done - helicopters, post-war jets etc. Most of their current mods will be going into the new UltraPack version too.