We're happy to have API clients, crawlers, scrapers, and browser extensions, but they have to obey some rules:
X-Ratelimit-Used: Approximate number of requests used in this period
X-Ratelimit-Remaining: Approximate number of requests left to use
X-Ratelimit-Reset: Approximate number of seconds to end of period
<platform>:<app ID>:<version string> (by /u/<reddit username>)
User-Agent: android:com.example.myredditapp:v1.2.3 (by /u/kemitche)
See JSON for an overview of some of the response types.
Changes to the API can happen without warning if necessary, subscribe to /r/redditdev for announcements of changes.