For stock iOS devices, you really have only 4 choices:
1) Ad Hoc distribution to up to 100 total max devices per iOS Developer enrollment (including wireless Ad Hoc via manifest file & SSL.)
2) Enterprise distribution for distribution to employees of corporations with a D&B rating.
3) Apple's iTunes App store if your app is approved by Apple. (This includes the B2B program and account/password protected apps.) (This now also includes up to 1000 people using Apple's new Testflight service.)
4) Unlimited distribution to other people who have their own individual, company or enterprise iOS/Apple Developer enrollments. The distribution can be either as an Xcode project with source code or a pre-compiled library, or as an ipa or archive file that the customer can (re)codesign with their own Developer certificates. For applications priced at well over $99 per customer, the cost of this annual developer program enrollment might only be a slight additional cost to the customer (and given appropriate legal authorizations, might even be handled as an annual paid service.)
4 b.) ADDED UPDATE: As of Apple's release of Xcode 7 (in late 2015), anyone with just a free Apple ID can use Xcode 7 on their Mac to install apps from build-able Xcode projects directly to their own tethered iOS devices this way, with no need to pay $99 to Apple to enroll. See this answer.
This essentially allows unlimited distribution to anyone with physical access to a current Mac and who knows how to run Xcode.
Options (1), (2) and (4) do not require going through App store approval. There are no other options for distributing apps to stock OS iOS devices.