Modelsim is sort of an industry defacto standard.
Xilinx ISIM is for Xilinx parts only and is still not as good as Modelsim
Incisive is a Linux/Unix based simulator from Cadance
Aldec is a another PC based simulator that competes directly with Modelsim on features.
VCS is synopsys' simulator which is very fast and is Unix based (funny how Google is reporting that their website has Malware on it)
Icaraus if you can't afford to pay for a "real" commercial simulator (don't know if there is a free VHDL simulator)
I use pretty much whatever my job provides, but I like Modelsim as I've used it more than all the others and am most familiar with it.