Streaming in a web browser
You are probably looking for Subsonic. With Subsonic, you can stream your music in a web browser, and even on your mobile. Here is an online demo.
Subsonic needs Java to run, so I hope you have a 512 Mb RPi (maybe a 256 is enough...) Please have a look to this RPi.SE question to know how to install Java on your RPi. Here are the instructions to install Subsonic on a Debian system (such as Raspbian). Maybe you should consider to move to Raspbian.
Please note I have never tried to install Subsonic on my Pi: I don't know if it works on it. If you manage to run it, please let us know by leaving a comment ;)
Ampache is an alternative to Subsonic, and is maybe more suitable for a little device like the RPi since it is written in PHP. Furthermore, it is available in the official repo, so you can install it with the following command:
$ sudo apt-get install ampache
If you plan to stream your music inside your own private network, you can use a DLNA server on your Pi, and a DLNA client.
MiniDLNA is server software with the aim of being fully compliant with DLNA clients.
To install MiniDLNA:
$ sudo apt-get install minidlna
Then, configure MiniDLNA by editing
Mediatomb has some additional such as a web interface for administration, on-the-fly transcoding (for devices that doesn't support the original codec), dynamic video thumbnail creation, ... and more here. But it has more RAM footprint than MiniDLNA.
To install Mediatomb:
$ sudo apt-get install mediatomb
DLNA clients (for Mac)
I don't have a Mac but VLC does support DLNA. Here is a SU.SE answer:
Go to VLC main menu and click View → Playlist and scroll down to Local Network. Locate Universal Plug and Play and click on it.
A few moments later your media service should be shown on the main window to the right. If you click on it (or double click) then it will start showing you all the folders/files being served. Give it time as it took mine almost 30 seconds to show everything.