Here's a little list of apps/sites I've found useful in staying on top of my business workflow.
Finances / Accounts / Invoice
Wave - Wave accounting app
This is my favourite of the online solutions I know of. Slick, built in solution from proposal through to payment recieved notices. It can notify you of over-due invoices, of clients to chase over quotes etc. Plus it has payment options built in. I've had clients comment on how easy I am to pay compared to some others at times from using this one.
Cushion - https://cushionapp.com/
I've only recently be using this one but the concept to me is solid. You can plan out your work (or at least see where you're over-committing and where you're not). Some of the functions overlap with Wave for me but it still seems to have some use for those with multiple clients or those that suffer the "rollercoaster" work schedule.
"F&&k you pay me" - Sorry about the curse but it's from a Creative Mornings talk by Mike Monteiro
The basics of making sure you get what you are owed. Firstly maybe watch the talk (yes it involves the F-bomb so maybe headphones if you have others near-by).
Second I've been told about this service which is maybe only useful if you keep having the issue. Basically http://justtelljulie.com/ will call those clients for you and use her "Thermonuclear Nice" approach at getting you not only what you're owed but also more work.
Ideas / projects for "down-time"
Things gone a bit quiet work-wise and you need something to keep you sharp? Check these guys out
Basically a whole bunch of example briefs that should get you thinking and making.