The only ones I have is a very small collection of MPCs (Military Payment Certificates) but I eventually want to collect some Confederate currency as well. I'm holding off on Confederate until I know more about them.
I collect intaglio items (typically steel engraving), which can run a pretty wide spectrum. I'm very taken with 19th century notes, the "golden age" of engraving. This include obsolete/broken banknotes, bank checks, vignette proofs, stocks and bonds, even a few Confederate notes. I love the work on the early federal currency, but since it's beyond my financial reach I collect souvenir cards which were printed by the BEP from the original plates.
Several years ago, I started collecting currency printed by the American Bank Note Company. Prices have significantly increased since I started collecting but most are still affordable. Later I started collecting notes that I like even if they were not printed by ABNCo. Currently, I'm looking for revolutionary currency.