The Canon EF-S 18-55mm f3.5-5.6

Is the Canon EF-S 18-55mm f3.5-5.6 compatable with the Canon XSi? I have the opportunity to buy a new one for relatively cheap.

It is, if its the 18-55 III (the one with IS) is the one that comes with the XSi as the kit lens

Yes.

EDIT:

ruben00 wrote in post #7953531
It is, if its the 18-55 II (the one with IS) is the one that comes with the XSi as the kit lens

IS version would be the third one out as there were two versions of the non-IS 18-55mm.

what lenses do you currently have?...the lens your currently have may be better...

anyt canon lens that starts EF, or EF-S is compatible with your XSI

ruben00 wrote in post #7953531
It is, if its the 18-55 III (the one with IS) is the one that comes with the XSi as the kit lens

Depth wrote in post #7953533
IS version would be the third one out as there were two versions of the non-IS 18-55mm.

No. The 18-55 IS is a completely different lens. It is in no way a variant of the earlier 18-55s. It is essentially just a coincidence that it shares the same focal length and aperture range as its predecessors.

Note also that there are 3 versions of the 18-55: a Mk 1, a Mk 2, and one with USM. All share the same poor optics and none have IS.