Question: I have multiple site collections.

```
/
/sites/corporate
/sites/test
/sites/projects

```

I am looking for a way to navigate to the other site collections from each other and the root site collection.

How can I go about doing this?

Answer: 
You may define `New Heading` (Site Settings -> Look and Feel -> Quick Launch) with the required URL for other site collections URLs to be added to the quick launch menu.
If you use SharePoint 2013 you can use Managed metadata navigation to create custom navigation for your site collections.

Read this [blog post](https://www.nothingbutsharepoint.com/sites/eusp/Pages/SharePoint-2013-Navigation-using-Managed-Metadata-Term-Store.aspx) for details.