Actin crosslinking - depends what you actually mean. If you are talking about bundling it is bringing together actin filaments and assembly them into bundles by means of dual valency actin binding proteins (two actin bindnig sites) or poly cations (polyamines).
All manner of things bind actin. It is often not very interesting. If one protein or protein complex binds actin twice (or more; much more in the case of myosin II) then it changes the mechanical properties of the actin meshwork.
Uncrosslinked actin is really weak, floppy and mechanically useless. Different types of cross linking give it strength and shape and allow it to push things.