C64 CCS64 (http://www.ccs64.com/ ) - this is a very nice Shareware emulator. You can try it for free as long as you like but if you plan to keep it then you should register your copy which currently costs 30US$. Note that Shareware writers produce very cheap products which are normally extremely well written so do remember they also need to eat!!
Plus 4 YAPE [Note Another Plus4 Emulator] (http://yape.homeserver.hu/download.htmhttp://yape.homeserver.hu/download.htm ) - Another fine shareware product. The author if this one simply asks for a donation of your choice. This one also emulates the C16 which shares much of the software with the Plus 4.
C16 Winemu (http://winemu.emuunlim.com/ ) - This one appears to be Freeware. It emulates the C16 and C64.
VIC20 VICE (http://www.viceteam.org ) - This one is definitely Freeware and emulates the VIC20 as well as the C64, Plus4 and the earlier Commodore Pet.