When I started learning about mastering, I was using T-Racks. People talk shit about it, but it really helped me understand the EQ/compression relationship that I think people need to learn when they're getting into mastering. It's not a bad plug-in if you're doing home demo stuff.
Then, I discovered the Waves plug-ins and noticed a big difference in the quality of the compressors and EQs. I haven't used all the Waves plug-ins, but the ones I have used have been really nice. I suggest
the Renaissance Maxx bundle and/or the Masters bundle. They have most of the plugs I've used.
Just this week, I took a leap into the world of the UAD-1 with the Project PAK.
The Pultec Pro EQ really changes the way I think about mastering. It's got a definite coloured tone, but I really like it and can see myself using it for most projects. I'm sure the UAD plugs will come in even more handy on my next mixing job, as many of the Project PAK plugs are more mixing-based than mastering-based. I assure you, I'll be spending the money on the Cambridge and Fairchild plugs when I can afford them.
The PSP Vintage Warmer is a fairly harsh sounding compressor. I really like it, but think it should be saved for the recordings that need a bit more dirt in their tone.
I just got a job as an ME at a pro studio here in Toronto, so I'm looking forward to getting used to the outboard gear I could really only dream about until I got a job like this. They also have the Sony Oxford plugs there, so I'll try and report on their sound when I get familiar with them.