I've spent about 10 hours so far this week looking at around 20 different cloud accounting services. I've read most of the related Quora forums and some other online reviews. My tentative conclusion is that Xero.com will the best for our startup, which has about 10 employees.

[For the asker, it looks like Xero does not support Paychex right now, but it has more add-ons than its competitors.]

LessAccounting.com, WaveAccounting.com, and Kashoo.com are also cool-looking double-entry accounting sites. They seem just a step below Xero and QBO on sophistication, but simplicity can also be a pro. Those are the ones I'll be comparing Xero with.

Sites like InDinero.com and Outright.com, which function like Mint.com, might be cool for freelancers, but they don't have true double-entry accounting, so no-go for most startups.

Expensive ($400+/month) and complex services like IntAcct.com or FinancialForce.com probably aren't needed until you're a big (50+/100+ people?) company.

Other sites I looked at include:
- Bill.com
- MarketCircle.com
- Harvest (getharvest.com)
- Expensify.com
- Freshbooks.com
- Shoeboxed.com
- Invoicera.com
- FreeAgent.com
- WorkingPoint.com
- Zoho.com
- Saasu.com
- NetSuite.com