There is a basic membership which limits the number of monthly searches, costing $4.99 a month, and an unlimited membership that costs $9.99.
You can also pay annually and save a little.
There are more benefits to the full membership which although I have, I cannot remember what exactly they are...
Other than seat alerts you can check what booking classes are left on each flight at any given time (and on LY that includes redemption classes).
In addition you can see live seat maps which work until the very last moment with some airlines (including LY) and with others up until around 24 hours prior to departure (like with BA for example) which is probably when airport have control of it.