1)PSP Vintage Warmer (which has already been mentioned). This thing rocks and does a really nice tape saturation emulation (as well as full on compression and limiting) which would probably do the trick quite nicely, especially for synth basses, etc. I've used the PSP VW quite often, with good results, on everything from bass and drums to guitars and synths.
2) UAD1 Fairchild compressor. Absolutely love it. This thing is amazingly versatile and has a truly wonderful warm sound. If you own a UAD it is totally worth it as it makes almost anything just sound better
