After getting annoyed* with the lens cap that my Sigma lens (Sigma AF18-200mm) came with, I replaced it with this. It is much easier to put back on, and it has a hole for a strap (to attach it to the camera body). The only downside is that the supplied string is a single thread (not looped, which would make attaching it much easier) and very very thin.
*Was sometimes difficult to put back on (especially when using a lens hood) and (being used to using bridge cameras with attached lens hoods) I sometimes forgot to put it back on at all.