Common words:
* costs
* lasts
* posts
* tests
* hosts
* rests
* bursts
* wrists
* lists
* casts
Less common words:
* abstracts
* amulets
* attests
* blasts
* crusts
* distended (the "ts" is in the middle of the second syllable)
* fists
* ghosts
* lusts
* mists
* pasts
* pests
* quests
* roasts
* rusts
* trusts
* watts
* yachts
Words with "ts" as part of a larger sound:
* Christmas (the "ts" is part of the "ch" sound)
* Christmases (the "ts" is part of the "ch" sound)
* cats (the "ts" is part of the "k" sound)
This is just a small selection, there are many more words with "ts" in the middle!