Short on money? Luckily, there are many excellent free plug-ins available, either made by small, amateur developers, or bigger ones that think it's good advertisement to release some stuff for free. All in all, you can now have a huge top-quality plug-in collection for free, without any of the guilt associated with piracy.
Every week, the music tech site Musicradar releases a list of free music software for that week. They've been doing it for a year and a bit, which means they now have at least 500 plug-ins rounded up. Articles are per week, but don't worry about starting with the last ones, they're recent enough not to be obsolete.
Music Radar free music software round-up
Then there's KVR Audio, a site dedicated almost exclusively to music production software. They have an amazing database of free and commercial plug-ins and other software, that you can easily access here: KVR Audio Database Search
I will add articles on free plug-ins that I think are special enough to merit their own article when I come across them as well, but these two sites are great starting points. On a final note, I think everybody should check out the amazing Independence Free, a free sampler with a 2.5 GB sound library, and for anyone wanting to know what plugins to look into, KVR Audio also has these ranking lists.