Ifranview and
Photoscape are my two favorites and freeware/shareware.
http://www.irfanview.com/Ifranview will run most Photoshop filters in .8bf format using a free utility also on the Ifranview site. There are links on the Ifranview site to free filters as well.
Although I purchased two others;
Corel PSP latest - more powerful potential however longer learning curve, takes much longer time to load each image file, render etc.
Serif - did not impress me so much after finding Ifranview.
Picassa is quick and easy to learn but more limited than the above.
Gimp
http://www.gimp.org/ is an open source free platform with the power of Photo Shop (I'm told) but harder to learn.
You can find links to many more on various photography forums as well.