There are a lot of computational methodologies from algebraic topology that you can apply here, moving from less to more complicated. Suppose E_{*} and E^{*} is a pair of a generalized homology theory and its cohomology theory, which has a commutative and associative product, and you have a space X where you are interested in the loop space ΩX and the free loop space LX, which live in fibration sequences ΩX -> PX -> X and ΩX -> LX -> X. (Here PX is contractible.)

There are Atiyah-Hirzebruch spectral sequences

H_{p}(Y; E_{q}(*)) => E_{p+q}(Y)
H^{p}(Y; E^{q}(*)) => E^{p+q}(Y)

which, because they are generic, do not have such stellar behavior except in "easy" cases.

If you have a good grip on the (co)homology of X, then there are the Serre spectral sequences associated to the path-loop fibration

H_{p}(X; E_{q}(ΩX)) => E_{p+q}(*)
H^{p}(X; E^{q}(ΩX)) => E^{p+q}(*)

and those associated to the free-loop fibration

H_{p}(X; E_{q}(ΩX)) => E_{p+q}(LX)
H^{p}(X; E^{q}(ΩX)) => E^{p+q}(LX)

The Serre spectral sequence is sometimes less-than-spectacular for loop spaces and free loop spaces, again because it's pretty generic, and because to use then to compute for the loop space you have to play the fiber off the base. This leads to nasty inductive arguments.

Then there are the Eilenberg-Moore spectral sequences for ΩX. If E^{*}X is a flat E^{*}-module and X is simply connected, then you get a spectral sequence

Tor_{**}^{E*X}(E^{*},E^{*}) => E^{*}ΩX

where this is Tor of graded modules over a graded algebra and inherits a bigrading. This is usually much more straightforward than the standard technique of playing the Serre spectral sequence game to find the homology of the fiber. There's also a homology version but it involves CoTor for comodules over E_{*}.

There's also an Eilenberg-Moore spectral sequence starting with Tor over the cohomology of X of the cohomology of LX with the ground ring, and converging to the cohomology of ΩX. This is often less useful because usually you want to go the opposite direction, but it exists.

Finally, there is the Hochschild homology spectral sequence for LX. If E^{*}X is a flat E^{*}-module and X is simply connected, then there is a spectral sequence

HH^{E*}(E^{*}X,E^{*} X) => E^{*}LX

where this is Hochschild homology of E^{*}X (over the ground ring E^{*}) with coefficients in itself. This is a graded algebra over a graded ring and the Hochschild homology recovers a bigrading. If you instead took coefficients in the ground ring E^{*} you're recover the Eilenberg-Moore spectral sequence for the based loop space ΩX.

For example, if E^{*} X is a polynomial algebra over E^{*} on classes in even degree, the cohomology of the loop space is exterior and the cohomology of the free loop space is the de Rham complex. More complicated cohomology yields more complicated behavior.

If you have specific spaces in mind then there are more specialized results. For example, one major theorem is the Atiyah-Segal theorem relating the K-theory of the classifying space of a compact Lie group to a completion of its complex representation ring. This is very hard to extract from the above general methods.

(Somebody who is an expert in string topology should step in and talk about K-theory of free loop spaces!)