I came across a new poem form recently, the sevenling. Roddy Lumsen , a contemporary Scottish poet, invented it. As the name suggests, it's made up of seven lines: three stanzas, the first and second stanza with three lines, and the third stanza with only one. Stanzas 1 and 2 should each contain "an element of three" -- statements, descriptions, etc. -- and Stanzas 1 and 2 do not have to relate to each other. Stanza 3 comments on the rest of the poem.
This Wikipedia entry has a wonderful poem by the Russian poet Anna Akhmatova that apparently inspired Roddy to make it a set form.
I gave it shot. The last line's not right, but that's all I've got right now.
It's a fun exercise -- give it a try, and post yours in the comments!