Some of the most useful server commands;

/sethome Set a 'home' to warp too
/home Return to the home you set
/back Go back to the most recent place you warped from

/msg "name" "message" Tell a player a private message
/r "message" Respond to the last respond to private message you

/warp Get a list of places you can warp
/warp mall Warp to the mall

/money View how much money you have
/pay "name" "amount" Pay a player some of your money
/town deposit "amount" Deposit money into your town bank

/t spawn Warp to your town spawn
/t outpost Warp to your towns #1 outpost
/t outpost 2 Warp to your towns #2 outpost

Town mayor and Assistant commands
/t claim Claim a 16x16 chunk
/t unclaim Unclaim a 16x16 chunk
/towny map View a map of surrounding towny claims
/t buy bonus "#" buy bonus claims for your town in the amount of #, each cost 100rings
/t assistant add "name" Promote a resident to the rank of assistant

For more informations please see the Towny Site