Most notably, it would be great to have a filter to search for a check number in the Transaction search tab (under "Fundraising > Transactions").  Whenever I enter a check, I add the check number.  Sometimes we don't have much information to search for besides the check number.  Other ideas for filter options may be to choose a type of transaction (credit card/cash/check/etc.), or even fund type (donation vs. registration fee, etc.)