Shure PG27USB and PG42USB mics as well as the X2u XLR to USB signal adaptor.
If you've already got a good mic that you want to use with your computer, sans mixing board, the X2u is probably the way to go. It sells for about $129 and includes an integrated preamp with a mic gain control and headphone jack for zero latency monitoring. It also supplies phantom power for condenser mics.
The PG27USB and PG42USB mics plug straight into your computer's USB port, no adapter or mixer required. Each has a headphone jack for zero latency monitoring. The PG27USB sells for around $199 and is designed for recording instruments or vocals with a flat frequency response. It comes with a stand mount and a carrying pouch, while the PG42USB comes with a shock mount and is designed for vocal and spoken word recordings. It has a tailored frequency response for vocals and costs about $249.