To store your materials online, so you can access them on any computer, try Dropbox! It's free!
My favorite therapy game is Pop-Up Pirate. It has 24 swords, which means up to 24 opportunities for data collection... or more if you require the child to produce his sound 5 times for each sword (or read 5 sight words, answer 5 comprehension questions, recall 5 details, etc.). It's good for groups since there are 4 different sword colors.
Flippin' Frogs also allows for a LOT of data collection! There are 24 frogs in 4 colors so it can also be used for groups. After the child earns all of the frogs, we turn on the rotating tree and see how many of them stay on the tree when it flips up.
Another fun game that allows for several trials per turn is Honey Bee Tree.
An obvious must-have is a good laminator. I like my Scotch Laminator more than the professional one at my clinic! I buy 5mm laminating pouches rather than 3mm as they are thicker and more durable.
Everyone needs some Play-Doh, markers, crayons, twistable colored pencils (no more sharpening!), and bubbles!
I also love these 3/4" binding rings for keeping my printed flash cards together as a set!