Hi Alex,
There are a number of options and approaches for this depending on your requirements:
1. Reuse the GwtTree2 that ServiceNow uses elsewhere in the system: Cool UI Page
2. Create your own entirely server side in Jelly (will have performance problems on large trees or deep nesting)
3. Create your own using some client side script or framework you import in (jsTree or angular.js for instance)
I have done #2 and #3 in the past, never tried #1 because I can't control if ServiceNow drops GwtTree in favor of something else. I can control scripts that I import. Use ServiceNow's tree picker as an example or the UI Page in the link above. ServiceNow loads the first level of the tree and then loads subsequent levels only one node at a time as you expand the nodes. This is best for performance on larger trees.
The HTML for these trees can be done either with a <table> or <ul><li> format. So really, you are just running an initial GlideRecord to get the first tier. Then each time a '+' is clicked, if the node hasn't been loaded, run a GlideRecord query to get all records with the parent you just clicked.
I hope that helps. Let me know if you need more information.
