The Gallium3D LLVMpipe driver is capable of running some old OpenGL games on the CPU at low resolutions assuming your processor is powerful enough.
LLVMpipe is commonly being used as a fallback now for composited Linux desktops in cases where no OpenGL hardware driver is available. LLVMpipe ends up shooting the drawing commands over to the CPU while it makes things faster than the Mesa classic software rasterizer by leveraging LLVM for CPU and multi-core optimizations. LLVMpipe is enough to handle Unity or the GNOME Shell if running a modern desktop x86 CPU, but obviously it won't come close to being comparable to a discrete GPU.
For kicks and in seeing how far the Intel Core i7 4900MQ "Haswell" CPU can be pushed, we tested the eight-thread processor on the System76 Gazelle Professional laptop. These results shouldn't really come as a surprise if you have been through my other Phoronix LLVMpipe articles, but I ran these tests for some Friday afternoon fun and in seeing how the current LLVMpipe performance on Mesa 9.3-devel Git with LLVM 3.3 compares to using the Intel Haswell graphics driver on this Core i7 CPU with HD Graphics 4600.
All the benchmark results in full can be found on OpenBenchmarking.org within 1308236-SO-LLVMPIPEM08. From that link are all the system details, logs, and Linux OpenGL benchmarks in full.
Embedded below are just some preview results for this Core i7 4900MQ LLVMpipe vs. Intel Haswell graphics comparison with the latest LLVM and Mesa from Xubuntu 13.10.