There are number of free as well as paid software to stitch together tiles out there. What is your intended purpose?
Home use or commercial?
computer screen/website or print?
If you are looking to make makes for commercial use, you will likely have to purchase software to get a good wide selection of usable elements. In this case, ProFrantasy has a decent backlog of stuff in their CC3 product line, especially if you include the annuals. Fractal Mapper has some some less than that. Also, you can find some stuff all over the web, but most of it is licensed for non commercial use.
Now, if you are just using for home use, then your options of "concent" go WAY up. Dundjinni's forums, rpgmapshare.com, etc.
Check out the software forum for a sticky. Lot's of stuff in there to look through. Ultimately, you could use a good simple graphics product such as GIMP, PhotoShop, or whatever else as long as it supports some type of snap to grid feature.