Definitely Dropbox.
Syncs a folder structure on your computer and also has a mobile app.
Get a good scanner that does double sided and scan everything.
I also use mobile capture to scan if I don't want to go to the computer.
I use it for more than just receipts. Loan agreements, contracts, warranties, etc
Even for spreadsheets and stuff I can update on the go.
I throw out the originals after scanning.