The tools depends a bit of the kind of work to do. For most browsing I go for DbVisualizer, I can customize it exactly to my needs. The support is good and responsive. For pl/sql editing I prefer oracle sqldeveloper.
DbVisualizer is not a free tool but the price is fair, maybe even cheap when compared to Toad. It is also multi database but this is implemented in such a way that I am not limited in my oracle world and it helps me navigating on other vendor rdbms that I don't know so well.
For real time monitoring DbVisualizer has a very nice option that I have not seen in any other application. VERY usable if you are trying to debug some, for example, performance problems.
Ronald