My fourth WordPress Plugin has the name Enable RAW HTML – Disable Corruption. It switches all the filters for your content off. Everything you type will be sent to your visitor’s browser. The filters are wp_autop, convert_chars, wptexturize.
If you need the same functions not only for posts and pages but also for excerpts or something else, have a look in the source and edit what you need. You can even enable PHP execution if you want.
There are many plugins like this, but this is the most effective. Now you get what you type.