Generalized symmetric algebras and Dickson algebras over ${\mathbb F}_p$. Start with the really well-known fact that $R[x_1, \ldots, x_n]^{S_n}$, where $R$ is any commutative ring, is polynomial on elementary symmetric polynomials.  Now consider the slight generalization of multiple collections of variables, namely $R[x(i)_1, \ldots, x(i)_n]^{S_n}$, where $i$ runs over some finite indexing set and $S_n$ still acts by permuting subscripts.  These rings are generally not polynomial algebras, in particular when $R$ is ${\mathbb F}_p$.
Ten years ago, in the context of computing the cohomology of symmetric groups, Mark Feshbach gave generators and inductively-defined relations for these rings when $R$ is ${\mathbb F}_2$.  My questions are:
(1) Does anyone know of calculations over ${\mathbb F}_p$ or other approaches over ${\mathbb F}_2$?
(2) Restricting to $R = {\mathbb F}_p$ and replacing $S_n$ by $GL_n({\mathbb F}_p)$ we get the Dickson algebras in the case of one collection of variables.  Has anyone studied the analogues of Dickson algebras where there are multiple collections of variables?
 A: Four quick references that contain substantial info on your questions (for more, it'd be good to know what exactly you would like to know...):
de Concini, C.; Procesi, C.
A characteristic free approach to invariant theory.
Advances in Math. 21 (1976), no. 3, 330--354. 
Grosshans, F. D.
Vector invariants in arbitrary characteristic. 
Transform. Groups 12 (2007), no. 3, 499--514. 
Stepanov, S. A.
Vector invariants of symmetric groups in the case of a field of prime characteristic. Discrete Math. Appl. 10 (2000), no. 5, 455--468 
Stepanov, Serguei A.
Orbit sums and modular vector invariants. Diophantine approximation, 381--412,
Dev. Math., 16, Vienna, 2008.
A: The paper [P. FLEISCHMANN, A NEW DEGREE BOUND FOR VECTOR INVARIANTS OF SYMMETRIC GROUPS,  TRANS. AMS
Volume 350, Number 4, April 1998, Pages 1703-1712] shows that this ring is generated by homogeneous invariants whose degree does not exceed max{n, k(n − 1)} (where i runs over an index set of size k).
Also this bound is sharp if $n=p^s$ for some prime $p$ and either $R=\mathbb Z$ or $R$ has characteristic $p$.    
Some work has been done on the Dickson invariants version as well.  I think that is considered in the article [Steinberg, Robert, On Dickson's theorem on invariants. J. Fac. Sci. Univ. Tokyo Sect. IA Math. 34 (1987), no. 3, 699–707.]
A: Do you mean the ring of diagonal invariants with $k > 1$?  This appears in the combinatorics literature (Garsia-Haiman and developments thereof) for $k=2$ and $R$ a field of characteristic 0, but the definition can be given for all $k$ and $R$.   It is just the $S_n$ invariants of the following object:
The polynomial ring (with $R$ as coefficients) generated by $nk$ variables, with the variables partitioned into $k$ disjoint sets of size $n$, and $S_n$ simultaneously (that is, "diagonally"), permuting all of the $n$-sets.
