On the cheaper end of the scale, it is quite easy to make a DIY "track" for use with any circular saw and/or router.

Just need a strip of 1/8" thick material ("hardboard", "masonite", or whatever they call it where you live), about 10" wide and as long as you want your "track" to be. This piece doesn't even need to be straight.

The second thing needed is a _straight_ length of something (eg. factory edge from 1/2" plywood, or more masonite, or..), perhaps 2-4" wide, which you glue to the top of the first piece, to become its "guide rail".

The circular saw (or router) then rides on top of the first piece, tight against the second piece. The first time it is used, it trims the first piece flush with its blade, creating a zero-clearance guide for future cuts.

Dirt cheap and easy to make, but it does normally require clamps to hold it in place when in use, which means it also then requires sawhorses or something to elevate it so the clamps can be used.

Terrible description, I know, but photo attached. Note that the saw in the photo is not the one I actually used with the guide (the original saw was larger), but it does illustrate how it would work.

EDIT: Here is an article showing the concept more clearly:
http://www.woodworkersjournal.com/making-straight-cuts-with-a-circular-saw/