A great blog that provides information and tips on how to streamline all the things you need to do Astronomy well. The wiki section is particularly useful as it contains a few guides for things like setting up a computer for Astronomy, data reduction, Python etc.
The now defunct Starlink project has a number of tools that are useful for astronomical data processing. SPLAT and PERIOD are two packages that are very useful for quickly looking at (and fitting lines of) spectra and performing timing analysis, respectively.
Want to know anything about any of the current or previous high energy missions, including where to get and how to process data? Go here. NASA's HEASARC website also provides the all important HEAsoft software package, including XSPEC (spectral analysis for X-ray telescopes), FV (fits file viewer) and FTOOLS (fits file utilities, including mission specific tools). You can browse mission data at http://heasarc.gsfc.nasa.gov/cgi-bin/W3Browse/w3browse.pl.
Data analysis threads for XMM and Chandra respectively.
The LaTeX cookbook - when you're writing papers you will have this website open almost constantly, I guarantee it.
If you have a Mac, it can be notoriously difficult to install one of the most important pieces of software for optical astronomers: IRAF. Use this page and it should see you through.
Great resource for IDL coding.
A list of optical/IR telescopes available to UK astronomers, including deadlines, how to submit proposals, instrumentation etc. Very useful.
For checking object visibility for your telescope proposal.
Sign up for your own personal daily digest of papers that ADS thinks are relevant/interesting to you.
Sign up to get involved in our weekly journal club discussion!
Latest arXiv papers distilled into a single bite-sized blog post!
The definitive list of astronomy conferences.
Daily short astro updates on things like transient events. Sign up for the daily email.
A compendium of other astro links, maintained by Dave Green
GitHub repositories, for version control and collaboration on software projects; and at this URL, you can have private repositories for free, if you subscribe with an academic email address.
There's also a "Student Developer Pack", with programming help, free domains, build checks, etc., here: https://education.github.com/pack.
Some Python-related useful links
An interactive Python 2D plotting library - bone up on this as you'll be needing it to make publication quality plots before you know it. For astronomy-specific things like FITS files, you may also want the Astronomical Plotting Library in Python (APLpy): http://aplpy.github.io/index.html.
Enthought Canopy, a very useful Python package manager and IDE.
Replace IRAF with Python.
Drive S-Extractor from Python.
Plot WCS axes from Python.
Replace the ugly matplotlib defaults.
For driving CasaPy from Python.
A range of other astronomy/Python-related packages.