General synonyms:
* Border
* Touch
* Connect
* Join
* Be next to
* Be contiguous to
* Lie beside
* Abut
* Be adjacent to
* Be contiguous with
* Share a boundary with
More specific synonyms:
* Attach (if referring to physical objects)
* Annex (if referring to adding a piece of land)
* Combine (if referring to merging two things)
* Interconnect (if referring to a connection between two things)
* Embrace (if referring to something surrounding another)
* Enclose (if referring to something surrounding another)
Informal synonyms:
* Next to
* Beside
* Alongside
The best synonym to use will depend on the specific context of the sentence.