Mage_Core_Model_Url::getUrl()

These are the core set of route parameters, you can be fairly sure that any
call to getUrl() which accepts route parameters will come through this method,
so any of these parameters should work.

|-----------------+---------------+---------------------------------------------------------------------------|
| Param | Type | Does |
|-----------------+---------------+---------------------------------------------------------------------------|
| _store | string, int | Use a stores config, numeric or string code |
| _type | string | Using the relevant types URL, either link, direct_link, js. media or skin |
| _secure | bool | Use the secure URL if allowed |
| _forced_secure | bool | Force using the secure URL |
| _use_rewrite | bool | Resolves the link to a rewritten SEO URL |
| _nosid | bool | Prevents the SID query param from being added |
| _absolute | bool | No effect as URLs are always generated as absolute. |
| _current | bool | Uses the current module, controller, action and parameters |
| _direct | string | Appended to the base URL, apparently (really?) |
| _fragment | string | Sets the fragment (#whatever) component |
| _escape | bool | Uses & instead of & for querystring parameters |
| _query | string, array | Sets the query parameters |
| _store_to_url | bool | Adds ___store to the query parameters |
|-----------------+---------------+---------------------------------------------------------------------------|

Mage_Catalog_Model_Product_Url::getUrl()

These are extra parameters only noticed by the product URL method.

|------------------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------|
| Param | Type | Does |
|------------------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------|
| _ignore_category | bool | Stops the category from appearing in the path. See Getting A Products URL for more info |
|------------------+------+----------------------------------------------------------------------------------------------------------------------------------------------------------|