[Robextend]

I echo Doug's post, and I add

1) Online Auctions - I would have never have been able to accumulate my collection without them.
2) Collector Forums - The knowledge I have gained and the friends I have made allow me to keep permanent interest in the hobby. If one collecting project ends there is always another one that can get started.
3) Other Resources - Baseball Reference, Old Cardboard, etc... Again, knowledge is power...
4) The ability to share your collection via websites...